This invention relates to a spraying control system for an electrically conducting liquid. This type of system can be used particularly in an inkjet print head using the continuous jet process.
In a liquid spraying control system with one or several continuous jets used in inkjet printers, each electrically conducting liquid jet is separated into drops. The drops are electrically charged and their path is then deflected by an electrical field which, depending on the information to be reproduced, deviates each drop either towards an ink recovery gutter, or to the support on which the ink is to be deposited.
In continuous jet printers, the ink is pressurized on the inlet side of a discharge nozzle. A continuous jet is discharged through the outlet from the nozzle. This continuous jet is processed by the liquid spraying control system by means of several devices performing a number of functions. Firstly the jet is separated into drops by a device controlled by a separating signal. At the same time, the drops separating from the continuous jet are electrically charged under the effect of the electrical field set up between the charging electrode and the liquid. They then enter a electrical deflection field generated between two electrodes or deflection plates to be deviated in this electrical field as a function of its value. At the exit from the liquid spraying control system, the ink drops are either recovered to return to the ink supply circuit, or are deposited on the support.
In practice, liquid spraying control systems used on printers have a number of disadvantages. They require that a large number of parts are made and positioned with high precision. These parts are complex and must be separated by xe2x80x9csafetyxe2x80x9d distances and/or shielding and by empty or insulating spaces that separate the functions, unnecessarily extending the path of the drops. Parts performing each function create discontinuous surfaces that cause internal local increases in the electrical field that facilitate electrical discharges. These surfaces are also difficult to clean when material residues are eliminated inside the print head. Since the parts performing each function are supported by insulators, their surfaces may become electrically charged in a variable manner and parasite electrical fields are then applied to the liquid. The result is random deviations of the drops. The electrical voltages involved with this type of control system may be as high as 10 kV.
In the current state of the art, drop deflections are frequently used in space at atmospheric pressure. Since the drops are electrically charged beforehand, a force is applied to them proportional to their charge and to the electrical field. This electrical field is obtained by two conducting plates close to the drop trajectory and to which a potential difference is applied. Document GB-A-2 249 995 proposes that one of the deflection plates could be coated with a dielectric coating to prevent accidental electrostatic discharges and/or to adapt the potential of the free space through which the drops pass. According to document U.S. Pat. No. 4,845,512, this dielectric coating may have a permanent electrical polarization (electret) in order to generate the electrical potential, or part of it. This would take place without any electrical connections. In this environment, the sprayed liquid and the presence of gas to which fairly intense electrical fields are applied create material particles and charges moving in free space. Spraying and the electrical forces drive these free elements (particles and charges) onto the walls located around the jet. In particular, these elements collect on the free surface of the insulation to which the electrical field is applied and are attracted by opposite charges. Thus, they compensate the charges of the electret or the electrode previously coated with insulation. Consequently, the useful field in the free space gradually reduces as the electrical field increases in the dielectric. The efficiency of the deflection reduces as a function of the reduction in the electrical field in free space.
Document WO 94/16896 recommends the use of electrically conducting plastic material to make a spraying control system for an electrically conducting liquid. This can reduce the cost, the number of ancillary parts such as shielding, and can simplify wiring. The plastic electrically conducting material also picks up the electrical charges. This plastic material may be made of polyacetylene which is an intrinsic conducting polymer. Preferably, it would be a plastic resin such as Nylon(copyright), polyester, acetal containing conducting fibers (carbon, stainless steel) coated with nickel. The heterogeneity of a fibrous resin increases at the surface, particularly for cast products. Since the insulating part of the fibrous plastic material is particularly on the surface, static charges can collect on the surface. Therefore the required conductivity effect reduces at the surface and deflection drifts occur as described in documents U.S. Pat. No. 4,845,512 and GB-A-2 249 995. Functional surfaces of the parts concerned can be machined to improve the surface homogeneity, but this increases their manufacturing cost.
Furthermore, the use of a volatile liquid in the composition of the ink causes condensation. Parts close to the inkjet gradually become coated with liquid, depending on internal ventilation in the printer, the partial pressures of the various surrounding gases and temperature gradients. This causes conduction phenomena on the walls of the deflection electrodes and a reduction in the space between the jet and the electrodes. A drift in the deflection of the drops is then observed during use of the spraying control system.
In order to overcome this problem, document U.S. Pat. No. 5,001,497 proposes to heat the deflection electrode concerned using an electrical resistance to vaporize the deposited liquid. The use of this type of resistance was criticized in document GB-A-2 249 995 due to the heat released by this resistance and due to the value of the current necessary for it to operate correctly.
One harmful phenomenon in these inkjet print heads is due to the possible interaction between drops in flight. A good spraying control system must have a short drop path to reduce this phenomenon.
Some manufacturers chose not to coat conducting deflection plates with a dielectric material. They include resistances in the deflection plate electricity power supply circuit in order to prevent accidental electrostatic discharges, in order to limit the discharge current in the circuit. Several types of electrical discharges may occur during operation of a printer.
The first type of discharge is given in the case of a voltage applied between two well polished plates. The electrical field is identical everywhere and shock ionization conditions take place uniformly on average. Thermal agitation causes a sudden increase in the current at a given moment that changes from an almost zero value to a gigantic value if there are no resistances in the circuit. The energy stored is used almost entirely within a very short instant depending on the form of the storage condenser, and this form defines the electromagnetic condition of the discharge transient. The dissipated power per unit volume is gigantic and is concentrated very locally. When metal plates are used connected to the high voltage power supply through about three meters of cable, the stored energy can exceed 1 mJ.
In other cases, electrical leaks are particular sources in space (conducting tips, insulation faults, foreign bodies) in which the field which is sufficiently strong locally generates an ion or electron source. The flow from this source is adjusted to a certain extent by means of the created space charge. The result is a stable current probably satisfying Langmuir""s law, and current fluctuations then occur with the current remaining finite. This second discharge case causes variations in the deflection field, and also variations in the charge on the drop. This reduces the precision of inkjet printers.
A known means of solving problems related to the first type of discharge is to place protection resistances, partly for safety of persons and the equipment (the electrode coating is subject to electro-erosion in the long term), and to eliminate the fire risk. These resistances must be located such that they compartmentalize the stored electrical energy. In particular, the energy stored in risk areas such as the deflection space must be reduced. The energy stored in this space is frequently of the order of 20 xcexcJ.
A first purpose of this invention is to reduce the number of mechanical and electrical components in a liquid spraying control system.
A second purpose of this invention is to eliminate discontinuities on internal surfaces of the liquid spraying control system.
A third purpose of this invention is to shorten the path of drops subject to interactions between each other in the liquid spraying control system.
A fourth purpose of this invention is to integrate the electrical circuits necessary for the liquid spraying control system into a single component.
These purposes are achieved by this invention which relates to a spraying control system for an electrically conducting liquid emitted in the form of a pressurized jet through at least one nozzle, comprising means of separating the liquid jet into drops, means of electrically charging the said drops and means of applying an electrical deflection field to the said charged drops, comprising:
two elements each with a continuous surface, laid out such that the continuous surfaces are facing each other and define a space between them in which the pressurized jet is emitted through the said nozzle, the said elements including means of continuously setting up potentials on the said continuous surfaces to obtain the said electrical charge of the drops and the said electrical deflection field,
electronic control means for the said potentials and means of checking the intensity of electrical currents that can circulate on the said continuous surfaces.
Advantageously, the continuous surface of the first element is conducting and has electrical connection means to one of the said potentials, the continuous surface of the second element is composed of one face of an insulating support, this face being equipped with conducting tracks with means of making electrical connections at potentials chosen among the said potentials, with a resistive coating with a resistance per square mm between 5 Mxcexa9 and 100 Mxcexa9, extending continuously over the said face.
The continuous surface of the first element may also be covered with a continuous resistive coating.
The first and the second elements may also be provided with means of separating the liquid jet into drops and inclining the jet. These means are used to apply an electrical field on the jet and may include resistive means and capacitive means. In this case, the resistive means are advantageously composed of a part of the resistive coating which preferably comprises discontinuities in some portions in order to increase the jet separation efficiency. Capacitive means may consist of the said coating supported by an insulating layer, this insulating layer acting as a dielectric and being supported by conducting means supported by the said insulating support.